The end pleats within the meaning of the invention are the two outer pleats on opposite end faces of the filter bellows. The end margins are the two free margins of the filter medium, which run along the end pleats and delimit same at the end faces of the filter bellows. The end edges of the filter bellows are the two other free margins of the filter bellows, which extend between the end margins and run so as to correspond to the pleating of the filter bellows. The pleated edges are the edges along which the filter medium is pleated. With commercially available, substantially cuboid filter bellows that are folded in a zigzag shape, the end margins and the pleated edges are generally straight and run parallel to one another. The end edges run in a zigzag shape and perpendicular to the end margins and to the pleated edges, when the filter bellows is viewed from the side. Before the filter medium is pleated, the end edges of the filter bellows—which later has a substantially cuboid imaginary envelope—run straight and parallel to one another. The imaginary envelope is stretched by the end edges, the pleated edges or end margins adjacent to a post-filtration side of the filter bellows, and the pleated edges or end margins adjacent to a pre-filtration side.
With filter bellows of flat filter elements, the filter media are not closed, i.e., the end pleats are not connected to one another, nor are the end edges. In contrast, with filter bellows of round filter elements, the filter media are closed, i.e., the end pleats thereof are connected to one another. Filter bellows of flat filter elements may be planar, but may also have bends in different directions.